Self-antagonistic drive for medical instruments

ABSTRACT

A medical instrument including a shaft and an actuated structure mounted at a distal end of the shaft can employ a pair of tendons connected to the actuated structure, extending down the shaft, and respectively wound around a capstan in opposite directions. A preload system may be coupled to maintain minimum tensions in the tendons.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent document is claims benefit of the earlier filing date ofU.S. provisional patent application 61/721,988, filed Nov. 2, 2013,which is hereby incorporated by reference in its entirety.

BACKGROUND

Instruments for minimally invasive medical procedures can be directlymanipulated manually or can be operated with computer control orcomputer assistance. However, computer manipulation of a medicalinstrument often places strict mechanical requirements on the medicalinstrument. In particular, the mechanical systems of a robotic medicalinstrument may need to have a tightly controlled response to actuatoroperation, so that a computerized control system can calculate actuatormovement that will achieve a precise movement of the instrument.Actuator controlled medical instruments may also need docking structuresthat engage electronically controlled actuators. For these reasons andothers, medical instruments that are suitable for computer assistedoperation tend to be cumbersome or difficult to use manually.

FIG. 1 schematically illustrates a medical instrument that may be usedin a robotic system for a minimally invasive medical procedure. (As usedherein, the terms “robot” or “robotically” and the like includeteleoperation or telerobotic aspects.) Instrument 100 includes a tool orend effector 110 at a distal end of a shaft 120. End effector 110includes jaws 112 and 114 that are rotatably mounted. Jaw 112 isconnected to a first pair of tendons 121 and 122, and jaw 114 isconnected to a second pair of tendons 123 and 124. Additional tendons(not shown) may be connected in instrument 100 to a wrist mechanism orjoints (not shown) that provide additional degrees of freedom forpositioning and orienting end effector 110.

Tendons 121, 122, 123, and 124 apply forces and torques to jaws 112 and114 when pulled by a backend mechanism 130 attached to the proximal endof shaft 120. Backend mechanism 130 may act as a transmission thatconverts the rotation of drive motors (not shown) into movement oftendons 121, 122, 123, and 124 and end effector 110. As shown, backendmechanism 130 includes one capstan 131, 132, 133, or 134 per tendon 121,122, 123, or 124, and the proximal ends of tendons 121, 122, 123, and124 respectively wrap around capstans 131, 132, 133, and 134 and thenattach to preload systems 135, 136, 137, and 138. Preload systems 135,136, 137, and 138 can be biased, e.g., include stretched springs, sothat non-zero forces are applied to the proximal ends of respectivetendons 121, 122, 123, and 124 through the full range of motion of endeffector 110. With this configuration, when capstans 131, 132, 133, and134 are free to rotate, the corresponding preload system 135, 136, 137,or 138 provides tension and avoids slack in tendon 121, 122, 123, and124.

End effector 110 can be operated using drive motors that are under theactive control of software executed in a controlled system thatinterprets human input (e.g., through master control input in amaster-slave servo control system). In particular, four drive motors,which are provided in a docking port of a control system (not shown),can be respectively coupled to rotate capstans 131, 132, 133, and 134.Backend mechanism 130 may dock with an interface of the control systemincluding motors or other actuators. When backend mechanism 130 isremoved from the dock, handheld operation of backend mechanism 130 maybe difficult particularly because of the shape of backend mechanism 130and the accessibility of capstans 131, 132, 133, and 134 and becausecontrol of each degree of freedom of end effector 110 involves using twocapstans, e.g., capstans 131 and 132 or 133 and 134.

SUMMARY

In accordance with an aspect of the invention, a medical instrumentincluding a shaft and an actuated structure mounted at a distal end ofthe shaft can employ a pair of tendons connected to the actuatedstructure, extending down the shaft, and respectively wound around acapstan in opposite directions to provide a self-antagonistic drivesystem. A preload system may be coupled to maintain minimum tensions inthe tendons.

One specific embodiment of the invention is a medical instrumentincluding an actuated structure mounted at a distal end of a shaft in amanner that permits movement of the actuated structure relative to theshaft. Two tendons, which may be opposite ends of a continuous cable orsimilar structure, may connect to the actuated structure and extend downthe shaft. A portion of one tendon may be wound in a first directionaround a capstan at a proximal end of the shaft, and a portion of theother tendon can be wound around the capstan in a second direction thatis opposite of the first direction. A passive preload system may becoupled to maintain tension in the tendons.

Another specific embodiment of the invention is a method for operatingan instrument that includes an actuated structure mounted at a distalend of a shaft. The method includes driving rotation of a capstan thathas a first tendon wrapped around the capstan in a first direction and asecond tendon wrapped around the capstan in second direction that isopposite to the first direction. Distal portions of the first and secondtendons extend along the shaft and engage the actuated structure. Themethod further includes passively controlling tension in proximalportions of the first and second tendons that extend from the capstan.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a known medical instrument that can berobotically controlled during a minimally invasive medical procedure.

FIG. 2 shows a medical instrument with a control system using a singlecapstan with oppositely wound actuating tendons respectively connectedto independent preload systems.

FIG. 3 shows a medical instrument with a control system using a singlecapstan with oppositely wound actuating tendons connected to a preloadsystem including a spring loaded movable pulley.

FIG. 4 shows a medical instrument with a control system using a singlecapstan with oppositely wound actuating tendons connected to a preloadsystem including an in-line spring system and a fixed pulley.

FIG. 5 shows a medical instrument with a control system using oppositelywound actuating tendons connected to a capstan in a spring-loaded, slidemounting.

FIG. 6 shows a medical instrument with a control system using a singlecapstan with oppositely wound actuating tendons respectively connectedto independent preload systems and additional take-up pulleys betweenthe capstan and an actuated structure.

FIG. 7A shows a medical instrument with a control system using a singlecapstan with oppositely wound actuating tendons on two independentone-way clutches/bearings connected to independent preload systems.

FIG. 7B shows a medical instrument with a control system using a singlecapstan with oppositely wound actuating tendons on two independentone-way clutches/bearings connected to a preload system including anin-line spring system and a fixed pulley.

FIG. 8 shows a system for control of two degrees of freedom of a medicalinstrument.

FIG. 9 is a perspective view of a backend of a catheter system with onlya single capstan for each degree of freedom that is compact enough forhandheld use and also capable of use with computer-aided control.

Use of the same reference symbols in different figures indicates similaror identical items.

DETAILED DESCRIPTION

A drive system for a medical instrument can employ a single motor drivencapstan on which two actuating tendons are oppositely wound forself-antagonistic drive of an actuated structure such as an endeffector. In general, an antagonistic drive can actuate a degree offreedom using two drive cables or tendons respectively connected to pullin opposing directions. With one type of antagonistic drive, the twotendons connect to two independent drive motors or actuators that arerespectively associated with opposing directions of a single degree offreedom. However, with self-antagonistic actuation as described herein,the two cables associated with opposite directions of a degree offreedom can connect to the same drive motor or actuator. As a result, aself-antagonistic drive system can employ one motor or actuator perdegree of freedom of an actuated structure, allowing the drive system tobe simpler, lower cost, and more compact than a drive system using onemotor or actuator per actuating tendon. Further, a self-antagonisticdrive system can be suitable for both handheld and robotic operation.One or more preload systems can maintain tension in the actuatingtendons, even when drive motors or actuators are off, which alsofacilitates in allowing the handheld or robotic operation. In differentconfigurations, the drive systems can be connected to proximal ends ofthe actuating tendons, directly or through pulley systems, or can beconnected to a slide mounted motor or capstan.

FIG. 2 schematically illustrates a medical instrument 200 in accordancewith an embodiment of the invention. Instrument 200 includes an actuatedstructure or steering section 210 at a distal end of a main tube 220,which is connected to a backend mechanism 230. Steering section 210 inthe illustrated embodiment includes flexible tubing 212 and an actuationring 214. Steering section 210 may, for example, include tubularvertebrae that are interconnected by joints or alternatively a tube ofan elastic material such as Nitinol having kerfs cut to create flexures.Steering section 210 may additionally include sheathing that covers thejoints or flexures. Ring 214 may be a rigid structure having actuatingtendons 221 and 222 coupled to opposite edges of ring 214, so thatpulling on tendon 221 or 222 tends to bend flexible tubing 212 in onedirection or the opposite direction of one degree of freedom of motionof ring 214. In a typical embodiment, a second pair of tendons (notshown) may be connected to actuation ring 214 for actuated movement inanother degree of freedom for movement of ring 214. Many otheralternative embodiments of steering section 210 are possible. Forexample, steering section 210 could include multiple independentlyactuated joints, and tendons 221 and 222 may be employed to actuate oneof those joints. Also, tendons 221 and 222 may be opposite ends of acontinuous structure such as a cable that winds through ring 214 oraround a particular joint in steering section 214.

Tube 220 may be a rigid or flexible tube but is generally less flexiblethan steering section 210. In particular, main tube 220 may besufficiently flexible to follow the path of a natural lumen. However,for steering of main tube 220, a backend mechanism 230 can applydifferent forces or tensions to tendons 221 and 222. The desired resultof the applied forces is bending of steering section 210 in a directionof the steering and minimal bending of tube 220. To achieve this goal,main tube 220 may be more rigid than steering section 210, or eachtendon 221 or 222 may be a Bowden cable, e.g., a pull wire enclosed in ahousing, which will minimize the bending of tube 220. Tendons 221 and222 can otherwise be stranded cables, wires, rods, or tubes made ofmetal, a polymer, or other material. In an exemplary embodiment, tendons221 and 222 may include connected portions of different construction,e.g., stranded cable that are fused to tubes. For example, the strandedcable may be used where significant bending or flexing of the tendons221 and 222 is expected, and the more-rigid tubes may be used elsewhereto reduce stretching of tendons 221 and 222.

FIG. 2 illustrates an example instrument in which actuating tendons 221and 222 attach to steering section 210, but alternatively actuatingtendons 221 and 222 could be used to operate other types of actuatedstructures such as a jaw as shown in FIG. 1, another type of jointedstructure, or any other mechanisms that permits movement of mechanicalmembers of a medical instrument. For example, tendons 221 and 222 coulddrive a pivot, planar, cylindrical, or spherical rolling joint orflexure that provides clockwise and counterclockwise rotational freedomto a jaw or other structure or drive a prismatic linear joint or slidethat provides linear bi-directional freedom of motion to an actuatedstructure.

Backend mechanism 230 attaches to the proximal end of tube 220 and actsas a transmission that converts the rotation of a drive motor 250 intomovement of or tension in actuating tendons 221 and 222. Backendmechanism 230 particularly manipulates tendons 221 and 222 to operatesteering section 210. In the illustrated embodiment, backend mechanism230 includes a capstan 235 around which portions of both actuatingtendons 221 and 222 are wound in opposite directions. For example,tendon 221 may be wound around capstan 235 so that counterclockwiserotation of capstan 235 reels in more of tendon 221 from the side oftendon 221 leading to steering section 210, and tendon 222 may be woundaround capstan 235 so that counterclockwise rotation of capstan 235feeds out more of tendon 222 toward steering section 210. Movement ofsteering section 210 back and forth along one degree of freedom can thusbe actuated through rotation of a single capstan 235.

Drive motor 250 is connected to rotate capstan 235, and in someimplementations, capstan 235 is an extension of or is part of the shaftof motor 250. In some other implementations, drive motor 250 has adetachable connection to capstan, so that backend mechanism 230 may beseparated from motor 250. Motor 250 may be under the robotic controlbased on human input (e.g., master control input in a master-slave servocontrol system) and software executed in a robotically controlledsystem. Additionally, a knob, lever, or other hand-operated manipulator260 is coupled to capstan 235 or motor 250, and enables a user tomanually operate instrument 200 through manual rotation of capstan 235.Instrument 200 may thus be used with or without motor 250 or knob 260applying a torque to capstan 235.

In various embodiments, a preload system 240 can be employed to maintainminimum and equal tension in tendons 221 and 222, avoiding slack intendons 221 and 222 as well as biased motion in steering section 210even when neither motor 250 nor knob 260 applies a torque to capstan235. Preload system 240 can be passive such that the applied tensiondoes not need to respond to a control or feedback system. In otherembodiments, preload system 240 can be actively controlled (e.g.,applying tensioning when a minimum tendon tension or slack is detectedor maintaining a predetermined tendon tension or tension range). In theembodiment of FIG. 2, proximal ends of tendons 221 and 222 extend fromcapstan 235 to preload system 240. In particular, each tendon 221 or 222may wrap around capstan 235 for a set wrapping angle (that could be lessthan a full turn or include more than one turns) around capstan 235, andthe proximal ends of tendons 221 and 222 extend past capstan 235 toconnect to preload system 240. Tendons 221 and 222 are not required tobe permanently attached to capstans 235 and thus may be able to sliprelative to capstans 235, for example, when motor 250 or knob 260 turnsin a direction that feeds tendon 221 or 222 out toward steering section210. However, the wrap angle and the tension applied by preload system240 are such that when motor 250 or knob 260 pulls in from the distalend of tendon 221 or 222, the torque applied by motor 250 or knob 260controls the tension in the distal portion of that tendon 221 or 222.

Preload system 240 in the embodiment of FIG. 2 is implemented usingsprings 241 and 242 that may be anchored to a case or chassis of backendmechanism 230. Springs 241 and 242 may be biased, e.g., stretched, toapply non-zero and equal forces to respective tendons 221 and 222throughout the range of motion of surgical instrument 200. With thisconfiguration, when capstan 235 is free to rotate, springs 241 and 242respectively pull on tendons 221 and 222 and control the tensions intendon 221 and 222. Preload system 240 may thus prevent slack in tendons221 and 222 by pulling in the required length of tendon 221 or 222.Further, preload system 240 applies an equal amount of tension on bothtendons 221 and 222 to avoid biased motion in steering section 210throughout the range of motion of surgical instrument 200.

Each spring 241 or 242 in preload system 240 more generally can bereplaced with any structure or system that is able to apply a force tothe free proximal end of a tendon 221 or 222 while allowing the requiredrange of displacement of the proximal end of the tendon 221 or 222.Springs 241 and 242 can, for example, be linear coil springs, constantforce springs, or use other spring elements, such as rotary coilsprings, leaf springs or compliant members, such as bending beams,cantilever beams, or elastic bands. Springs 241 and 242 can be any typeof compliant members, springs, or tension-applying systems, but thetension that spring 241 applies may ideally be equal to that applied byspring 242 throughout the range of motion of the instrument. Otherwise,the preload on each tendon may be unbalanced, creating biased motion atthe steering section. Further, the spring elements or compliant memberscan work through extension or compression to apply force directly orindirectly to the end of the attached tendons. In addition, the springelements or compliant members may be designed so that the force appliedby spring 241 on tendon 221 is equal to the force applied by spring 242on tendon 222 throughout the range of motion of the instrument 200.Other methods for applying the desired force, such as a system usingweights or magnets, might alternatively be employed. In addition to thesource of force, preload system 240 may include mechanical elements (notshown) that direct or control the magnitude of the force applied to theattached tendon, e.g., to apply a constant force throughout the range ofmotion of steering section 210.

FIG. 3 illustrates an example of an instrument 300 including a catheterhaving a main tube 220 and a steering section 210 that can be controlledusing tendons 221 and 222 wound around a capstan 235 in a backendmechanism 230 as described above with reference to FIG. 2, butinstrument 300 differs from instrument 200 by employing an alternativepreload system 340. Preload system 340 includes a biased spring 342attached to a pulley 344 having a slide mounting that permits pulley 344to move toward or away from capstan 235. In preload system 340, theproximal ends of tendons 221 and 222 are connected together and loopedaround pulley 344. In operation, drive motor 250 or knob 260 can rotatecapstan 235 to increase the tension in a distal portion of tendon 221 or222 and cause steering section 210 to bend or move toward the highertension tendon 221 or 222. At the same time, spring 342 allows pulley344 to shift and rotate until the proximal ends of both tendons 221 and222 carry tensions about equal to one half of the force that spring 342applies to pulley 344. As described above, the tension in the distalportion of tendon 221 or 222 being pulled in will depend on the motor ormanual torque applied to capstan 235, and the tension in distal portionof the tendon 222 or 221 being reeled out will be about the same as thetension at the proximal end. Preload system 340 can thus maintainnon-zero and equal tensions in the proximal ends of tendons 221 and 222,avoiding slack in tendons 221 and 222.

FIG. 4 illustrates another example of an instrument 400 including a maintube 220 and a steering section 210 that can be controlled using tendons221 and 222 wound around a capstan 235 in a backend mechanism 230 asdescribed above with reference to FIG. 2, but instrument 400 includesanother alternative preload system 440. Preload system 440 includes anin-line spring 442 and a fixed pulley 444. Fixed pulley 444 can beanchored to the walls, case, or chassis of backend mechanism 230. Theproximal end of one tendon 221 or 222 connects to one end of spring 442,and the proximal end of the other tendon 222 or 221 connects to theother end of spring 442 after wrapping around fixed pulley 444. Spring442 is biased, e.g., stretched, to apply equal tension to the proximalends of tendons 221 and 222, and spring 442 has a sufficient range ofmotion to compensate for stretch that may occur in tendons 221 and 222and axial compression of main tube 220 or steering section 210. Motor250 or knob 260 can control a higher tension at the distal end of onetendon 221 or 222 as described above, while preload system 440 controlsthe minimum tension in both tendons 222 and 221.

FIG. 5 illustrates still another example of an instrument 500 alsoincluding a main tube 220 and a steering section 210 that can becontrolled using tendons 221 and 222 wound around a capstan 235 in abackend mechanism 230 as described above with reference to FIG. 2.Instrument 500 includes a preload system 540 that spring loads capstan235, instead of connecting directly to tendons 221 and 222. In FIG. 5,preload system 540 is coupled to drive motor 250, and drive motor 250has a slide mounting that permits linear movement of motor 250 andcapstan 235 in a direction perpendicular to the rotation axis of capstan235. Alternatively, preload system 540 could couple to capstan 235 inanother manner, e.g., to knob 260, bearings (not shown) of capstan 235,or to a slide mounting (not shown) of capstan 235. In the embodiment ofFIG. 5, the proximal ends of tendons 221 and 222 can be attached to orfixed on capstan 235.

FIG. 6 illustrates yet another example of an instrument 600 alsoincluding a main tube 220 and a steering section 210 that can becontrolled using tendons 221 and 222 wound in opposite directions arounda capstan 235 in a backend mechanism 230 as described above withreference to FIG. 2. Instrument 600 may also include a preload system240 that is the same as preload system 240 of FIG. 2 or an alternativepreload system such as described with reference to FIG. 3, 4, or 5.Instrument 600 differs from instrument 200 of FIG. 2 in the addition oftake-up spring system 640 that engages tendons 221 and 222 betweencapstan 235 and steering section 210. Take-up system 640 in FIG. 6includes pulleys that respectively engage tendons 221 and 222 and arespring loaded to pull on tendons in a direction perpendicular to thelengths of tendons 221 and 222. Take-up system 640 thus provides anothermechanism for maintaining non-zero tension and avoiding slack in tendons221 and 222 regardless of which direction capstan 235 turns.

The tendon 221 or 222 being fed out may need to slip on capstan 235 inorder for the passive preload system to maintain at least the minimumtension at all times in the distal portions of tendons 221 and 222. Inanother implementation, two tendons in a self-antagonistic drive systemwrap in opposite directions around two independent one-way clutches orbearings. The one-way clutches can be oriented with opposite senses, sothat only one clutch engages per drive rotation direction. FIG. 7A, forexample, illustrates a self-antagonistic system 700 in which two tendons721 and 722 respectively wrap in opposite directions around respectiveone-way clutches or bearing 731 and 732. One-way clutch 731 is orientedso that clutch 731 pulls on the distal side tendon 721 when motor 250 orknob 260 drives a central shaft 730 clockwise, and clutch 731 slips whenmotor 250 or knob 260 rotates shaft 730 counterclockwise. One-way clutch732 is oriented so that clutch 732 pulls on the distal side tendon 722when motor 250 or knob 260 rotates shaft 730 counterclockwise and slipswhen motor 250 or knob 260 rotates shaft 730 clockwise. Thus, only oneclutch 731 or 732 will be engaged for each drive rotation direction. Apassive preload system 740 of system 700 may eliminate the chance ofslack or tension build-up in tendon 721 or 722 due to the stretch of theother tendon 722 or 721 because passive preload system 740 can pull inslack on the free-wheeling clutch 721 or 722.

The mechanism of the preload system 740 may be identical to the preloadsystem 240. As shown, proximal ends of tendons 721 and 722 connect tospring systems 741 and 742 in preload system 740. Spring systems 741 and742 maintains minimum and equal tensions in tendons 721 and 722,avoiding slack in tendons 721 and 722. Alternatively, any other preloadsystem such as those described herein could be employed. FIG. 7B, forexample, is identical to FIG. 7A except that the tendons 721 and 722connect to a preload system 745 including an in-line spring system 743and a fixed pulley 744 in the same manner as preload system 440 of FIG.4.

Steerable instruments as mentioned above can benefit from the ability tocontrol the pitch and the yaw of the distal tip of the instrument. FIGS.2, 3, 4, 5, and 6 shows some examples of medical instruments in a distaltip, e.g., steering section 210, can be bent back and forth to controlone angle, i.e., pitch or yaw, of the distal tip. More generally, amedical instrument could contain two such drive system for independentcontrol of the pitch and yaw angles of the distal tip. FIG. 8, forexample, illustrates a medical instrument 800 employing two pairs ofactuating tendons 221 and 222 and 221′ and 222′ having distal endsconnected to a steering section 210 of a medical device such as asteerable instrument. For example, the distal ends of tendons 221, 222,221′, and 222′ may all be connected to an actuation ring 214 at 90°separations around the perimeter of ring 214. Tendons 221 and 222 windin opposite directions around a capstan 235 that has a preload system240 for tendons 221 and 222. Tendons 221′ and 222′ similarly wind inopposite directions around a capstan 235′ that has a preload system 240′for tendons 221′ and 222′. Pitch and yaw angles of the distal tip ofinstrument 800 can thus be controlled using two motors 250 or knobs 260coupled to capstan 235 and 235′.

The separation of tendons 221 and 222 and the separation of tendons 221′and 222′ at ring 214 may be perpendicular to each other for pitch andyaw actuations. As a result, associated drive systems, particularlycapstans 235 and 235′, may also be perpendicular to each other. Theperpendicular orientations may not be the best configuration for acompact drive system for convenient handheld use of instrument 800.However, the orientation and position of drive system components such ascapstans 235 and drive motors 250 can be rearranged using a pulleysystem 852 or a drive transfer systems 854. In particular, pulleysystems 852 can be used to redirect tendons 221, 222, 221′ and 222′ sothat capstans 235 and 235′ do not need to be perpendicular. Drivetransfer system 854, e.g., a belt or gear system, can similarly be usedto change the position or orientation of either motor 250 relative tothe capstan 235 or 235′.

Motor 250 as shown in FIG. 8 does not need to directly attach to capstan235. More generally, a motor pack, which may include multiple drivemotors, e.g., motors 250 and 250′ in system 800, can connect tocapstans, e.g., capstans 235 and 235′, through an engagement mechanismthat allows the motor pack to engage or disengage the backend mechanismincluding capstans of a self-antagonistic drive system. Each manual knob260 may remain attached to the corresponding capstan 235 and applyhigher tension on one of the tendon 221 or 222 to steer the instrumentmanually. Removal of the motor pack from the backend mechanism hasadvantages. In particular, the removable motor pack may be outside asterile barrier that encloses a sterile area in which a medicalprocedure is performed. The motor pack may then be spared from thestandard but intrusive cleaning procedures such as high pressureautoclave sterilization that may be required for the backend mechanismand the rest of the instrument. If the backend mechanism is part of asingle-use instrument, the instrument can be easily replaced andrecycled while the motor pack can be used again and again. The motorpack can also remain or be permanently attached to a roboticallycontrolled arm, making the instrument smaller and lighter during manualuse. For example, once a physician is done with the handheld operationof the instrument, the physician can attach the backend mechanism of theinstrument onto the motor pack and robotic arm. An input device (e.g., ajoystick) can then be used to control the instrument robotically.

One compact or small radius configuration of a drive system for aninstrument steerable in pitch and yaw directions orients rotation axesof drive motors 250 and 250′ and capstans 235 and 235′ along thedirection of main tube 220. FIG. 9 illustrates an instrument 900 inwhich drive motors 250 and capstans 235 are oriented along the axis ofmain tube 220. Pulley systems 910 and 910′ in instrument 900 can connectto respective passive preload systems 440 and 440′ and change thedirections of tendons 221 and 222 and tendons 221′ and 222′ that runalong that axis through main tube 220, so that the proximal ends oftendons 221 and 222 and tendons 221′ and 222′ are perpendicular to theaxis of main tube 220. Tendons 221 and 222 and tendons 221′ and 222′ canthus be wound around respective capstans 235 and 235′ as describedabove.

One specific embodiment of instrument 900, which can provide pitch-yawdrive, can be light weight, e.g., around one pound and compact, e.g.,have a maximum outside diameter less than about 60 mm. Main shaft 220can include four tendons or pull wires, two for pitch and two for yaw,terminated at the tip of the steering section 210 at the cardinalpoints. Each tendon may be a pull wire in a Bowden cable with the pullwire being distally terminated on a ring in the distal steering sectionand proximally terminated on a preload system that allows controlledsliding. Each pull wire may include a section of a polymer cable (e.g.Kevlar) that may be routed by idlers to the motor shaft or capstan. Thepolymer cable portion may also wrap or wind around on motor shaft, wheretwo sections that wind around the same motor shaft are wound in oppositedirections. The preload mechanism can keep minimum tension in the pullwires at all times.

The drive systems described above can provide significant benefits formanual and computer assisted operation of an instrument. In particular,for a biopsy, a surgeon or other medical personnel may want to manuallyinsert an instrument through a patient natural orifice such as the mouthor anus and the backend mechanisms, as described above, may be madesmall enough for handheld use during the insertion. One or twomechanical knobs can be provided for manual operation, allowing 2-way or4-way, bending of a tip section of the instrument. For example, theknobs can be oriented as in a standard bronchoscope or colonoscope. Themotor axis of the actuation motors can be parallel with the instrumentshaft, which may leave more room near the patient's anatomy for easiermanipulation. The relatively light weight and small visual mass of atleast some of the drive systems described above may also be appealing orless frightening to patients undergoing a procedure such as a biopsyunder conscious sedation, where the patient may be moving and aware. Forcomputer assisted operation, drive systems can use one motor or actuatorper degree of freedom, which may reduce cost and system complexity whencompared to a drive system using one motor per cable.

Although particular implementations have been disclosed, theseimplementations are only examples and should not be taken aslimitations. Various adaptations and combinations of features of theimplementations disclosed are within the scope of the following claims.

What is claimed is:
 1. A medical instrument comprising: a shaft; anactuated structure mounted at a distal end of the shaft in a manner thatpermits movement of the actuated structure relative to the shaft; acapstan that is rotatably mounted; a first tendon having a first portionconnected to the actuated structure and extending down the shaft to thecapstan, a second portion wound around the capstan in a first direction,and a third portion extending from the capstan; a second tendon having afirst portion connected to the actuated structure and extending down theshaft to the capstan, a second portion wound around the capstan in asecond direction that is opposite the first direction, and a thirdportion extending from the capstan; and a passive preload systemdirectly connected to the third portion of the first tendon and thethird portion of the second tendon, the passive preload systemcomprising at least one spring that is coupled to maintain tension inthe first tendon and the second tendon.
 2. The instrument of claim 1,wherein the passive preload system comprises: a first spring coupled toapply force to a proximal end of the first tendon; and a second springcoupled to apply force to a proximal end of the second tendon.
 3. Theinstrument of claim 2, wherein: the first spring comprises a constantforce spring that applies a constant force to the proximal end of thefirst tendon over a range of motion of the proximal end of the firsttendon; and the second spring comprises a constant force spring thatapplies the constant force to the proximal end of the second tendon. 4.The instrument of claim 1, wherein the passive preload system comprisesa pulley system engaged with the third portion of the first tendon andthe third portion of the second tendon.
 5. The instrument of claim 4,wherein the at least one spring applies tension to the first and secondtendons by pulling on the pulley system.
 6. The instrument of claim 4,wherein the at least one spring is coupled between the third portion ofthe first tendon and the pulley system.
 7. The instrument of claim 4,wherein the proximal ends of the first and second tendons are connectedtogether and wrap around a pulley in the pulley system.
 8. Theinstrument of claim 1, further comprising a motor coupled to rotate thecapstan, wherein the motor has an interface that enables computercontrol of the motor.
 9. The instrument of claim 8, further comprising amechanism that couples the motor to the capstan, wherein the mechanismallows the motor to be decoupled from the capstan for manual operationof the instrument.
 10. The instrument of claim 8, further comprising aknob coupled for manual rotation of the capstan.
 11. The instrument ofclaim 1, further comprising a second preload system that engages thefirst tendon at a location between the capstan and the actuatedstructure.
 12. The instrument of claim 1, wherein the passive preloadsystem applies the tension without need to respond to a control orfeedback system.
 13. The instrument of claim 1, wherein the at least onespring comprises a first spring directly connected to the third portionof the first tendon.